1. Field of the Invention
The present invention relates to electrical power conversion and, more particularly, to switching power supply devices.
2. Background Information
In the present state of the art, electrical power conversion development is directed to power converter designs which provide for increased monitoring functions and which can sustain increased operating power densities. As is well known, the power density of an electrical power converter can be improved by reducing the physical sizes of its power processing components: transformers, inductors, power transistors, and capacitors. In a switching power supply topology, for example, component size can be reduced by increasing the converter switching frequency. However, for low-voltage power supplies the primary factor limiting the operating power density is the efficiency of the power converter. Low-voltage, high-power power supplies find widespread application in electronic devices such as laser diode systems and computers operating high processing speeds. Non-isolated converters with synchronous rectification topologies may be used in such applications.
It is thus highly desirable to improve the efficiency of a power converter. Among the benefits realized, in addition to achieving a higher power density, is that the life of the converter is extended because there is less wasted power produced requiring dissipation. Most of the power requiring dissipation originates in the output rectifier, which may account for 50% to 75% of the waste power in the converter. Converters having moderate to high power densities are known in the art, but such converters typically require special application, additional heat sinks, or power derating for reliable operation.
In isolated power converter designs, forward and fly-back topologies use a direct drive scheme for synchronous rectification where the switching transistors are driven directly by the secondary of the power transformer. Although such configurations are relatively simple, they tend to be very inefficient because neither zero voltage nor current switching is attained during operation. Furthermore, forward and fly-back topologies do not efficiently utilize the magnetic core of the transformer (e.g., a forward converter topology will utilize only half of a component transformer core). Moreover, for synchronous rectifier control circuits, such as that exemplified by U.S. Pat. No. 5,956,245, "Circuit and method for controlling a synchronous rectifier converter," issued to Rozman, active clamps may be required.
Other power converter configurations derive a driving signal for control of the output switching transistors from the input side of the converter by capacitance coupling these input signals to the output signals. This approach increases the isolation capacitance and sacrifices a critical parameter of the power converter. In yet another configuration, phase-locked loops are used to synchronize the input and output switching frequencies, such as may be found in a control circuit which includes a synchronous rectification driver such as International Rectifier driver IR1175.
It is known in the prior art to configure a push-pull topology such that the master unit switches at a signal level 10% higher than the slave unit(s). In this topology, an oscillator output signal produced by the master PWM is used to drive the slave units. The oscillator output signal provided by the master PWM does not include a feedback error signal which modulates the ON time of the output feedback error signal which, in turn, modulates the ON time of the output drivers. Accordingly, output current sharing was neither possible nor predictable to any degree of accuracy.
A power supply system including a plurality of parallel connected power supplies is disclosed in U.S. Pat. No. 4,717,833, "Single wire current share paralleling of power supplies," issued to Small. Small '833 teaches that a master/slave scheme is not possible in redundant operation because of potential failure of the master unit. If the output of either master or slave shorts (due to the output capacitor failure), the so called redundant system is useless.
In the present state of the art, redundant systems have two or more identical power sources such that the inputs of the power sources share the same input power bus (i.e., all inputs are fused). The power source outputs are combined by means of OR-ing diodes or power transistors, and only one power source is on at a time. If one power source should fail, another power source takes over.
Therefore, there is a need for a DC-to-DC converter with high efficiency, high power density, output current sharing, and bidirectional control.